Earth oriented three axis stabilized satellites have generally no continuous yaw attitude information available or even no yaw measurement at all. This is true especially for commercial communication satellites which have to be designed under stringent economic conditions. The common approach to achieve 3-axis stabilization with a 2-axis attitude sensor only (Earth sensor) is to establish a bias momentum perpendicular to the orbit plane which leads to observability of the yaw motion by the roll measurement. An early publication in this field is e.g. [DoSR68], which is well-known as the "Whecon"-principle.
The invention provides a control design approach for a generalized Earth pointing control mode with 2-axis Earth sensor measurements only and bias momentum coupling, where time varying attitude reference signals with respect to an Earth pointing reference frame are considered. This means, the control method according to the invention addresses a tracking problem in addition to a disturbance rejection problem. A possible control task is shown in FIG. 1. The desired spacecraft attitude, here a roll-bias angle .alpha. and zero pitch- and yaw angles, can be expressed as a time varying reference attitude with respect to the orbital Earth pointing reference frame (x.sub.a y.sub.a z.sub.o). Furthermore, roll-tracking is necessary in case of inclined orbit operations of geosynchronous satellites, where proper Earth orientation for antenna pointing purposes has to be maintained.
Another example is small satellites in low Earth orbits, that use--besides the solar array rotation--one degree of freedom around the satellite yaw axis for the optimal orientation of the solar panels, i.e. to assure that the sun vector is always (nearly) perpendicular to the panel surface.
The subsequent explanations cover as far as possible a general case. Examples are given for the above mentioned application of roll tracking operations.
The minimum sensor and actuator hardware configuration which is necessary for the realization of this attitude control approach consists of the following components:
a.) A set of wheels that span the 3-dimensional space, i.e. practically linear actuators which produce torques around the 3 spacecraft axes. Usually 4 wheels are used for redundancy. PA1 b.) An Earth sensor that delivers a 2-axis attitude information around the roll- and pitch axis.
Additionally, an actual spacecraft has to be equipped with actuators for angular momentum control, such as magnetic torquers, thrusters, and/or solar array drives for solar torque compensation, depending on the spacecraft mission.